A CRT designed to reproduce color images generally includes a gun for generating three separate electron beams which converge towards the internal surface of the screen. Cathodoluminescent materials are deposited as phosphor stripes or dots on this surface and emit red, green or blue light when the respective electron beams impinge thereon. A perforated metal mask, spaced a very short distance from the internal surface of the screen, facilitates the selection of the colors so that the electron beams emitted by the gun only reach the corresponding phosphor elements, excluding all the others. The color selection mask requires a very precise shape and its distance from the internal surface of the screen guarantees the purity of the colors of the image formed on the screen. Only a small fraction of the electrons emitted by the gun (between 20 and 30%) are transmitted through the mask to reach the screen. The remainder impinge upon the imperforate portion of the mask and impart their energy to the mask, which heats up to a temperature of around 70.degree.-80.degree. C. This heating causes expansion of the mask material and modifies the position of the impact points of the electron beams on the phosphor screen elements, resulting in a loss of color purity as electron beams excite several phosphor elements of different emissive colors. Masks produced from sheets of low carbon steel are highly sensitive to this thermal expansion phenomenon. It is difficult with such masks to produce images of high luminosity or brightness without any loss of color purity.
European patent number 124354 proposes the use of an alloy with a low thermal expansion coefficient (e.g. iron/nickel), as material for the production of perforated masks. Nevertheless, this type of alloy possesses a strong tension or stiffness and the yield strength, at ambient temperature, makes it difficult to press-form the mask as the metal sheet (approximately 200 .mu.m thick) tends to return to its original shape if the yield limit is not exceeded. The solution envisaged by patent EP 124354 consists in pressing the mask from an iron/nickel alloy sheet at a temperature at which the value of the elasticity coefficient is lowest and, for example, close to that of mild steel, this temperature being about 150.degree.-200.degree. C. for a 35 wt % nickel 65 wt % iron alloy.
The mask forming temperature is generally obtained via a press whose punch and counter-punch, designed to give final shape to the mask, are heated to a temperature higher than that of the mask. The mask is then heated by convection, conduction and radiation in the chamber containing the punch and counter-punch. When the mask has reached the required temperature, it is then pressed by the punch. This prior shaping process has a number of drawbacks:
before being shaped, the mask material has its periphery in contact between the punch and counter-punch, and its center with the punch: in this configuration the transmission of heat is non-uniform.
heating by conduction and radiation requires bringing the press parts, which are to provide this heating, up to a temperature greater than the required temperature of the mask material, which entails a high consumption of energy and mechanical problems because it is difficult to maintain the moving parts of a press at high temperature.
the time required to bring the mask material up to the pressing temperature greatly increases the length of the shaping stage of the operation as compared to that of a mask made of mild steel and requires more mask-shaping, or pressing stations, to obtain a correct rate of production.
Alternatively, the temperature of the mask can be built up in a furnace outside the press; however, this involves problems of handling and loss of energy due to the low thermal capacity of the thin perforated sheet used for the mask. As described in EP 124354, the mask material can also be heated by immersion in an oil bath which involves problems both of handling and maintenance of the press's environment.